Delignification of cellulosic pulp with ammonia gas followed by heating in the presence of oxygen

ABSTRACT

A PROCEDURE FOR THE DELIGNIFICATION OF A CELLULOSIC PULP WHICH COMPRISES FIRST UNIFORMLY DISTRIBUTING AMMONIA GAS IN THE MOISTURE OF A FLUFFED PULP OF A CONSISTENCY OF ABOUT 15 TO BOUT 40% AND THEN, IN THE ABSENCE OF INTERMEDIATE WASHING, THE GASEOUS AMMONIA-TREATED PULP IS HEATED IN THE PRESENCE OF OXYGEN MAINTAINED AT A PRESSURE OF ABOUT 4 TO ABOUT 12 BARS AT A TEMPERATURE OF ABOUT 85 TO ABOUT 135*C. FOR A TIME OF ABOUT 10 TO ABOUT 360 MINUTES TO PROVIDE A PULP WITH A FINAL PH OF 9.6 TO 10.4.

United States Patent DELIGNIFICATION OF CELLULOSIC PULP WITH AMMONIA GAS FOLLOWED BY HEATING IN THE PRESENCE OF OXYGEN Norman Liebergott, Chometley, David W. Clayton, Hudson, and Raimbault M. A. T. de Montigny, deceased, by Madeleine C. de Montigny, Baie dUrt'e, Arthur de Gaspe Tache, Montreal, and Bernard W. Burgess, Senneville, Canada, executors, assignors to Pulp and .Paper Research Institute of Canada, Pointe Claire,

Quebec, Canada No Drawing. Filed May 24, 1971, Ser. No. 146,297

Int. Cl. D21c 9/10 US. Cl. 16265 5 Claims ABSTRACT OF THE DISCLOSURE A procedure for the delignification of a cellulosic pulp which comprises first uniformly distributing ammonia gas in the moisture of a flufied pulp of a consistency of about 15 to about 40% and then, in the absence of intermediate washing, the gaseous ammonia-treated pulp is heated in the presence of oxygen maintained at a pressure of about 4 to about 12 bars at a temperature of about 85 to about 135 C. for a time of about to about 360 minutes to provide a pulp with a final pH of 9.6 to 10.4.

BACKGROUND OF THE INVENTION (1) Field of the invention This invention relates to the delignification of a cellulosic pulp by the action of oxygen in an alkaline environment. More particularly, it relates to the treatment of a chemical cellulosic pulp in a moist comminuted (e.g. flufI'ed) form with ammonia gas followed by treatment with gaseous oxygen.

(2) Description of the prior art The brightening and delignifying action of air or oxygen on chemical cellulosic pulp in an alkaline medium has long been known. One major difiiculty in its use was the severe degradation of the cellulosic component of the pulp.

Attempts to provide a solution to this problem aredescribed in the following references:

(1) US. Pat. No. 3,251,730 issued May 17, 1966 to Sadayoshi Watanabe, which teaches a process for refining a digested pulp substantially free from chlorine. The procedure involves immersing the pulp in an aqueous medium of sodium hydroxide or sodium carbonate where the sodium hydroxide is a 0.1 to 2.0% solution and the sodium carbonate is a 0.5 to 4.0% solution. The pulp is then compressed to remove any excess aqueous medium so as to provide a wet mass of pulp having a pulp consistency of about 20% to about 66.7%. The wet mass is then loosened to provide a porous pulp mass. Finally the porous pulp mass is treated with an oxygencontaining gas at a temperature of from 60 C. to 100 C.

(2) US. Pat. No. 3,274,049 issued Sept. 20, 1966 to Marcel M. Gaschke et al. which teaches a procedure for the ammonium hydroxide-oxygen pulping of bagasse or similar non-woody, ligno-cellulosic materials. The procedure involves treating bagasse in an aqueous alkaline solution containing ammonium hydroxide as essentially the sole alkaline producing reagent, the solution having a pH of from about 9 to about 12.2. A free-oxygen-containing gas is passed through the aqueous alkaline solution, the partial pressure. of the oxygenlbeing from about 25 toabout 250 pounds per square inch. The aqueous reaction medium is maintained at a temperature of between about 130 C. and 150 C. for a digestion time of beice tween about one-half to one and one-half hours. The procedure is alleged to produce fibrous pulp in a yield not less than 40% by weight expressed on the basis of the dried and depithed bagasse.

(3) Canadian Pat. No. 743,847 issued Oct. 4, 1966 to Andre Robert et al., which teaches the delignification and bleaching of chemical and semi-chemical cellulose pulps, under the action of oxygen in an alkaline medium. The procedure involves the treatment of the pulp in the presence of a catalyst, used at a concentration from 0.5 to about 3% by weight on the pulp. The catalyst is barium carbonate, calcium carbonate, magnesium carbonate, zinc carbonate, alkali metal borates or titanium dioxide.

(4) US. Pat. No. 3,423,282 issued Jan. 21, 1969 to Paul Rerolle et al., which teaches a procedure for the two-stage delignification and bleaching of chemical cellulose pulps. The procedure involves first treating the cellulose pulps with oxygen in an alkaline medium at a temperature of from 80130 C. under less than 15 bars pressure for approximately 1 to 3 hours. Then the alkaline pulp is contacted with chlorine at a concentration of 2 to 4% by weight based on bone dry oxygen-treated pulp, at room temperature for not more than one hour.

(5) A paper entitled A Continuous Process for Oxygen Bleaching by Henry H. Myburgh, Paul Rerolle, and Johan Richter, presented at the 23rd Alkaline Pulping Conference, Florida, Oct. 14, 1969, which teaches a preliminary mild acid pretreatment of the pulp, performed with sulphurous or sulphuric acid at ambient temperatures for 18 to 60 minutes at pH 1 to 4, usually at a consistency of 3 to 5%. Then the pretreated pulp is subjected to the oxygen bleaching stage by introducing highconsistency (20%) pulp, intimately mixed with caustic soda and a pulp protector such as magnesium carbonate, into a pressurized reactor in a fluifed, or teased, condition. This allegedly results in an improvement in brightness with somewhat less degradation. The entire treatment exposes the fiber surfaces to a steam-oxygen atmosphere at pressures to 180 p.s.i. and temperatures no higher than 266 F. Retention time of the pulp fibers is controlled by a mechanical cascade system that causes sequential flow through the reactor.

DEFICIENCIES OF THE PRIOR ART When operating under conditions specified as above in the prior art, the elimination of lignin is nearly always accompanied by a signficant reduction in the mechanical strength of the cellulosic pulp. To attempt to minimize and alleviate this loss in strength, the prior art, as noted above, have offered an acid pretreatment prior to the oxygen bleaching stage, and the addition of an inorganic catalyst or viscosity protector (i.e. a material which prevents the reducetion of the viscosity of the pulp to a level at which pulp strength suffers) e.g., barium carbonate, calcium carbonate, magnesium carbonate, zinc carbonate, alkali metal borates or titanium dioxide. Magnesium carbonate in the preferred catalyst according to some of the prior art teachings. The alkali most commonly selected and added to the pulp prior to treatment with the oxygen is usually caustic soda, although one United States patent, namely US. 3,251,730 described above shows a preference for the use of sodium carbonate. The alkaline material is added to the pulp at a low concentration between 6 to 10%, then the pulp is pressed to a higher consistency, 20% in most cases, and then the pulp is treated with oxygen gas at a temperature ranging 6 bars for a retention time up to 3 hours.

3 SUMMARY OF THE INVENTION Aims of the invention An object of a broad aspect of this invention is to provide an improved process for the delignification of cellulosic pulp by the action of oxygen.

An object of another aspect of this invention is to provide such an improved process involving the exclusive use of gaseous reactants.

Broad statement of the invention According to a broad aspect of this invention, a process is provided for the delignification of a cellulosic pulp which comprises: substantially uniformly distributing gaseous ammonia preferably using air, steam or oxygen as a carrier gas in the moisture of a pulp comminuted into fiber and fiber aggregates and at a consistency of about 15 to about 40% usually until from about 1.5 to about 3.0% ammonia has been added, then heating the gaseous ammonia-treated pulp at a temperature of about 85 C. to about 135 C. (usually from about 85 C. to about 120 C.) in the presence of introduced oxygen maintained at a pressure of about 4 to about 12 bars (usually about 80 to about 100 p.s.i.g.), for a time of about to about 360 minutes (usually about 30 to about 180 minutes).

Description of other variants of the invention According to an embodiment of the present invention, process has been provided which makes it possible to eliminate the pretreatment acid stage and to substitute gaseous ammonia for caustic soda or sodium carbonate as the alkaline reagent. The process of an aspect of this invention has a great advantage whereby the pulp can be comminuted and fluffed into fiber and fiber aggregates at a consistency of about 15 to about 40% and the ammonia in gas form is directly added to the fi'uffed pulp using either air, steam or oxygen to provide a uniform distribution of the ammonia in the moisture of the pulp. The use of the ammonia, besides providing the alkaline environment, also eliminates the need for a viscosity or strength protector, such as, for example, magnesium carbonate. The pulp, after treatment with ammonia gas, is then heated in an autoclave at an elevated pressure and As used herein the term pulp comminuted into fiber and fiber aggregates includes fiuifed chemical cellulosic pulp (sometimes referred to as shredded chemical cellulosic pulp) and one example thereof is the product obtained by fragmentation of a chemical cellulosic pulp into fiber and fiber aggregates or flocks. Each discrete fiber aggregate or flock is spongy, compressible and readily gas permeable. Each of the fiber aggregates or flocks is usually of an average size of about A"1" in diameter and presents a fluffed-up, dry appearance even with about 40-85% by weight moisture in the cellulosic pulp. One preferred manner of flufiing or shredding the chemical pulp to the form above described is provided in Canadian Pat. No. 869,267 issued Apr. 27, 1971 and in Canadian Pat. No. 902,859 issued June 20, 1972.

DESCRIPTION OF EMBODIMENTS OF THE INVENTION The following are examples of aspects and embodiments of this invention.

EXAMPLES 1-20 Unbleached kraft pulp from coniferous wood, characterized by Kappa Number 32.4 and a CED viscosity of 36.5 cps. was pressed to 30% consistency. The pulp was then comminuted into fiber and fiber aggregates according to the process described in Canadian Patent application Ser. No. 068,411. Samples of such prepared pulp were placed into a stainless steel digester and treated in the following manner:

The pulp was treated with a mixture of ammonia gas diluted with steam until 1.5 or 3.0% ammonia based on oven dry weight of pulp was added to the pulp. Gaseous oxygen was added to the digester until a pressure of 80 or 100 p.s.i.g. Was obtained. The digester was heated and the pulps held at temperatures of 85 C. or 120 C. for reaction periods of 30, 60, 90, 120 and 180 minutes. After each treatment, the digester Was degassed and the pH of the pulp measured. The pulp Was then Washed, air dried and the Kappa Number and CED viscosity measured.

The treating conditions and the results obtained are shown below in Table I.

TABLE I Treating Conditions for Ammonia-oxygen Delignification of an Unbleached Kraft Pulp Kappa No. 32.4, CED Viscosity, 36.4, Pulp Consistency 30% N Ha,

percent 02 Reaction 0.5% CED on o.d. pressure, Temp., time, Final Kapp viscosity pulp p.s.i.g. 0. min. pH No. cps.

at an elevated temperature between about 85 C. and about C. Oxygen is introduced and the pressure usually is in the range of about 4 to about 12 bars (i.e. about 58 to about 174 p.s.i.g. and preferably about 80 to about 100 p.s.i.g.) and the pulp, after the appropriate retention time (about 10 to about 360 minutes, preferably about 30 to about 180 minutes) is washed. It may then be subjected to the actions of other bleaching agents, such as for example, chlorine dioxide, chlorine monoxide, peroxide, hypochlorite, etc.

The preceding examples can be repeated with similar success by substituting other specifically described reactants and operating conditions of this invention for those used in the preceding examples.

From the foregoing description, one skilled in the art can easily ascertain the essential characteristics of this invention, and without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions, Consequently, such changes and modifications are properly, equitably, and intended to be, within the full range of equivalence of the following claims.

We claim:

1. A process for the delignification of an unbleached cellulosic pulp which comprises: substantially uniformly distributing gaseous ammonia in the moisture of a pulp comminuted into fiber and fiber aggregates and at a consistency of about to about until about 1.5 to about 3% ammonia, based on the oven-dry weight of the pulp has been added to the pulp; then, in the absence of any intermediate water-washing step, heating the so formed gaseous ammonia-treated pulp at a temperature of about C. to about 135 C. in the presence of introduced oxygen maintained at a pressure of about 4 to about 12 bars, for a time of about 10 to about 360 minutes, to provide a pulp whose final pH after said heating step is about 9.6 to about 10.4.

2. The process of claim 1 wherein the ammonia gas is carried by a carrier gas selected from air, steam or oxygen to assist in providing the uniform distribution.

3. The process of claim 1 wherein the oxygen is maintained at a pressure of about 80 to about p.s.i.g.

4. The process of claim 1 wherein the heating temperature is about 85 C. to about C.

5. The process of claim 1 wherein the heating time is about 30 to about minutes.

References Cited ROBERT L. LINDSAY, JR., Primary Examiner A. L. CORBIN, Assistant Examiner U.S. Cl. X.R. 81 11 

